This invention relates generally to hydraulic cylinders and pistons, and particularly to a hydraulic piston having a piston rod threaded into the piston.
In the operation of any hydraulic cylinder and piston combination very severe mechanical stresses are present, for the hydraulic oil fluid pressures are typically upwards of 3,000 pounds per square inch (p.s.i.), and these pressures are applied across the piston surface in both directions. The piston rod transmits the force resulting from these high hydraulic fluid pressures to a driven member, and the rod is therefore subjected to compressive, tensile, and torquing forces. Unless the piston and rod are formed of one piece or welded together there must be a fluid seal and mechanical stress union between these components in order that they may properly function. Where a threaded connection is employed between the rod and piston, either by means of a piston-retaining nut on the threaded end of the rod or by threading the piston itself onto the rod, a torqued prestressing of this union is normally made in order to prevent premature fatique failure from the high reversing stresses that are experienced as the piston is hydraulically driven from one direction to the other. The present invention specifically relates to an improvement in hydraulic piston and rod assemblies wherein the connection is made by means of a threaded union.
Normally, the prestressing of the union between piston and rod is achieved by torquing the threaded members against a shoulder to effect a clamp load beyond the expected working stress range, but naturally lower than the initial failure limit of the weaker member. This shoulder is preferably a cross-axis load bearing shoulder between the piston and the rod of an area size sufficient to withstand the forces developed by hydraulic fluid pressure in a first piston direction. In the reverse piston direction the hydraulic fluid pressure forces are contained by the threaded piston/rod union, which threaded union is initially torqued to a prestressed tightness. To the extent that the threaded union is insufficiently tightened, operational failure of the piston and/or rod will be hastened by the reversing hydraulic pressure impact stresses, because any relative movement between these members will cause deformation, fatique and ultimate fracture. I have found that loosening of the threaded rod/piston union is less a result of relative rotation between these members than it is a result of natural self-centering tendencies which exist between the members. A tightened thread occurs between male and female threaded members when the angled thread pressure flanks, normally designed with sufficient clearance for relative rotation between male and female member, are wedged together to cause a mechanical clamp between the members. This process draws and frictionally holds one member off center from the axis of the other member to the extend of the normal diametric thread clearance. However, the high hydraulic pressure forces which reciprocate the piston and rod within the cylinder create working stresses which relieves the friction hold and tends to center the threaded members to a common axis, thereby reducing the prestressed threaded clamp load and allowing the failure process to begin.
The foregoing problem is compounded when larger pistons and rods are used, with their corresponding larger thread sizes. The running clearances required by threaded members are defined in industrial thread fit classes, and these clearances increase with increasing thread diameters, thereby increasing the degree of possible off-axis position of the rod when torqued tightly against the piston. Since the heavy torquing of one member relative to the other can draw the torqued member out of axial alignment with the other member by as much as the diametric clearance between threads, and since the force generated by hydraulic pressure is correspondingly greater with larger pistons, the natural self-centering tendency described above therefore occurs even more readily with larger pistons and rods.
I have discovered that the problem can be eliminated by the proper design of a centering pilot arrangement at either end of the threaded male member on the piston rod, which pilot arrangement holds the piston and rod in axial alignment regardless of the prestressing torque applied in tightening the rod to the piston. This dual piloting mechanism not only resists any rotational tendency that might be present between the rod and piston, but also it prevents any relative lateral motion between them over the entire threaded length of the union. The pretorqued, prestressed union between rod and piston is positively fixed and locked in all planes, creating a sealed mechanical, interference fit "X" bridge between corresponding ends of the threaded rod portion.
In the prior art it has been observed that the self-centering tendency described above, which loosens the piston relative to the rod, requires subsequent retorquing of the rod relative to the piston after work stressing. A number of these retorquings apparently stabilize the clamp load retention and ultimately results in a tight union between the piston and rod. While this is naturally a maintenance nuisance where the hydraulic cylinder is accessible to permit such retorquing, it is an impractical procedure for internal assemblies which necessitate removal of the hydraulic cylinder from its machine and disassembly of the cylinder to gain access to the piston and rod. Therefore most hydraulic cylinder assemblies are not attended to to correct this retorquing need until internal failure of some form takes place.
In the prior art, attempts at maintaining concentricity between parts have been resulted in the design of interfitting conical surfaces which are drawn together by means of some threaded locking mechanism. Such an apparatus is described in U.S. Pat. No. 551,913, issued Dec. 24, 1895, and modified as in U.S. Pat. No. 3,885,461, issued May 27, 1975. However, the use of such conical surfaces for this purpose requires that extremely close machine tolerances be adhered to, and virtually perfect fitting male and female cones be designed. This is difficult to accomplish under average manufacturing conditions and therefore results in very costly components.
Also in the prior art, lock nuts were frequently used to secure a threaded piston rod to a piston, and the pistons themselves have been at times threaded, with the threads usually extending throughout the piston length, to mate with a correspondingly threaded piston rod. Because of the increase in thread length over a piston lock nut, the possibility of the piston becoming loose on the piston rod is somewhat lessened although not eliminated. Resort has been made to various adhesives or bonding agents in an effort to prevent loosening, but the proper amount of bonding material to be used is difficult to achieve in actual practice. If too much bonding agent is applied it becomes exceedingly difficult to remove the piston when it becomes necessary to do so, and if not enough is applied then an imperfect lock is obtained. The curing time required by bonding agents has resulted in an imperfect locking, and hydraulic leakage passed the threads has frequently been encountered as a problem in this approach.
The prior art has approached the problem of sealing against hydraulic fluid leakage between the threaded surfaces in various ways. For example, in U.S. Pat. No. 3,187,645, issued June 8, 1965, a jam ring is compressibly tightened against the piston outer surface and deformed into engagement with the piston rod threads. This approach requires extremely high compression forces and a number of different components and uncertain assembly operations in order to be functional. Prior art devices have used relatively soft O-rings whose performance is dependent upon satisfactory control of the extrusion gaps adjacent the O-rings, for the hydraulic fluid pressure internal the cylinder generates large extrusion forces. These forces can easily deform poorly fitting O-rings and thereby destroy their sealing functions.